Starch, a complex polymer of glucose, is present in most green plants in practically every type of tissue and is the major intracellular reserve polysaccharide in photosynthetic organisms. The glucan accumulates during development of storage or seed tissues and is catabolized to serve as a source of energy. In the animal kingdom, as well as in fungi, yeast and bacteria, the primary reserve polysaccharide is glycogen. Glycogen is a polysaccharide containing linear molecules with α-1,4 glucosyl linkages and is branched via α-1,6-glucosyl linkages. Although glycogen is analogous to starch with regard to linkages, glycogen exhibits a different chain length and a different degree of polymerization.
The following common reactions are shared by the biosynthetic pathways of bacterial glycogen and of starch in algae and higher plants:ATP+α-Glc-1-PADPGlc+PPi  (1)ADPGlc+α-1,4-glucan→ADP+α-1,4-glucosyl-α-1,4-glucan  (2)Elongated α-1,4-glucan chain→Branched α-1,6-α-1,4-glucan  (3)
In step (1), ADP Glucose (ADPGlc) is synthesized from ATP and glucose-1-phosphate in the rate-limiting reaction which is catalyzed (in plants and bacteria) by ADP-glucose pyrophosphorylase (also referred to as ADPGlc PPase; or ADPG PPase, or glucose-1-P adenyltransferase, or as enzyme EC. 2.7.7.27). The chain elongation step (2) is catalyzed by glycogen synthase (also referred to a GS, gly A, or as enzyme EC. 2.4.1.21).
1. ADP-glucose phyrophosphorylase
The reaction scheme catalyzed by ADPGlc PPase is shown below: 
Significant research has led to cloning and sequencing genes which code for ADP-glucose pyrophosphorylase in plants for the purpose of modulating sucrose and starch content in plants. For example, U.S. Pat. Nos. 5,498,831 and 5,773,693 to Burgess et al. described the sequence of Pea ADP-glucose pyrophosphorylase subunit genes and the use of those genes to transform plant cells in order to provide plants that have increased sucrose content.
U.S. Pat. No. 6,184,438 to Hannah describes mutant genes encoding plant ADP-glucose pyrophosphorylases and the use of those genes to produce transformed plants having enhance germination characteristics but without any diminishment in food quality or flavor.
U.S. Pat. No. 6,057,493 to Willmitzer et al. describes the use of anti-sense DNA sequences encoding ADP-glucose pyrophosphorylase from potato to produce transformed plants with a reduction in starch concentration and an increase in the concentration of at least sucrose.
The genes which code for several bacterial ADPGlc PPases have also been cloned and recombinant enzymes have been prepared. See for example, Preiss, J., and M. N. Sivak, M. N. (1998) Genet. Eng. (N.Y.) 20, 177-223; Preiss, J. (1996) In Escherichia coli and Salmonella typhimurium: Cellular and Molecular Biology (Neidhart, F. C., Ed.), 2nd ed., Vol 1 pgs. 10115-1024, ASM Press; Preiss, J., and Romeo, T. (1989) Advances in Microbial Physiology 30, 183-238. DNA sequences have also been elucidated for certain bacterial ADP-glucose pyrophosphorylase enzymes. For example, U.S. Pat. No. 5,349,123 to Shewmaker et al. describes a nucleic acid construct which encodes an E.coli ADP-glucose pyrophosphorylase and the use of that construct to transform plant cells to modify biosynthesis of a glucan in the plant.
2. Glycogen Synthase
The reaction scheme in bacteria catalyzed by glycogen synthase is shown below: 
The “corresponding” reaction in mammals is similar except that the nucleoside diphosphate sugar substrate is UDP glucose, and the catalyzing enzyme is EC 2.4.1.11.
Genes encoding mammalian glycogen synthases have been cloned and sequenced (See Browner et al., Proc. Nat. Acad. Sci. (1989) 86:1443-1447; Bai et al., J. Biol. Chem. (1990) 265:7843-7848), and the genes encoding bacterial glycogen synthases have also been cloned and sequenced. (See for example U.S. Pat. No. 5,969,214.)